Microporous hydrophilic fluoropolymer membranes and method

ABSTRACT

An hydrophilic microporous diaphragm comprising a fluorine-containing substrate to which has been radiation co-grafted a mixture of monomers comprising at least one functional monomer of formula I, CF 2  ═CF(CF 2 ) n  A, formula II, CF 2  ═CF--O--(CFX--CFX) m  A, a dicarboxylic acid containing the group of formula III, --R 3  (COOH)--R 4  (COOH)--, and at least one non-functional monomer of formula IV, CY 2  ═CYZ, or formula V ##STR1## wherein the molar ratio of co-grafted functional monomer to non-functional monomer is in the range of 2:1 to 1:20. These diaphragms have enhanced wettability for improved performance in chlor-alkali cells.

TECHNICAL FIELD

This invention relates to a method of rendering hydrophobicfluoropolymers hydrophilic.

In particular the invention relates to a method of preparing hydrophilicfluoropolymers suitable for use as microporous diaphragms inelectrochemical cells, especially in cells electrolyzing alkali metalchloride solutions.

BACKGROUND ART

Electrolytic cells are commonly used to make chlorine and an alkalimetal hydroxide solution by electrolysis of an alkali metal chloridesolution. Such cells are known as chlor-alkali cells. Reference is madein this specification to chlor-alkali cells and processes as typifyingelectrolytic cells and processes in general.

There are three broad types of chlor-alkali cell, "mercury", "diaphragm"and the more recently developed "membrane" cells.

In membrane cells the anodes and cathodes are separated by cation-activepermselective membranes; these are membranes which are selectivelypermeable so as to allow the passage of only positively charged ions andnot the passage of bulk electrolyte. Cation-active perm-selectivemembranes which are suitable for this use in chlorine cells include, forexample, those made of synthetic organic copolymeric material containingcation-exchange groups, for example, sulphonate, carboxylate andphosphonate. Perm-selective membranes are non-porous.

On the other hand, diaphragm cells, in which the anodes and cathodes areseparated by porous diaphragms, permit the passage of both positive andnegative ions and of electrolyte.

In operating a diaphragm cell for the electrolysis of alkali metalchloride solutions to give chlorine and alkali metal hydroxides, it isessential that flow of the solutions through the tortuous microporousdiaphragm be unimpeded by gas voids in the porous network.

Diaphragms prepared from asbestos fibres have generally been used butthese suffer from the disadvantage that

(1) the lifetime of the fibrous asbestos network in the chlor-alkalicells is limited;

(2) the handling of asbestos fibres is often environmentallyundesirable; and

(3) the thickness of the fibrous matte limits the extent to which theinterelectrode gap can be reduced.

Alternatively diaphragms comprising fluoropolymer materials in sheet orin fibrous form which are inert to the cell liquors have been proposed.However, these diaphragms suffer from the problem that they arehydrophobic and difficult to wet with alkali metal chloride andhydroxide solutions and in consequence tend to have gas-filled voids inthe porous network of the diaphragm. This can lead to diaphragmblockage, high voltages and mixing of the product gases, hydrogen andchlorine.

Several methods have been proposed for rendering such diaphragmshydrophilic. For example UK Patent 1,081,046 and Belgian Patent 794,889to ICI Ltd describe processes for microporous sheet diaphragms in whicha hydrophilic particulate inorganic additive such as titanium dioxide isadded to confer hydrophilicity on the diaphragm matrix. Other additivessuch as surface active agents have also been proposed for this purpose.

These additives suffer from the disadvantage that

(1) they generally affect adversely the processing of the fluoropolymersinto a fibrous or sheet form;

(2) they are readily leached out by flow through the diaphragm; and

(3) initial wetting out of the tortuous microporous network is difficultto achieve satisfactorily.

It is an objective of the present invention to provide hydrophilicfluorocarbon membranes for use in chlor-alkali electrolysis cells.

We have now devised a process of rendering hydrophobic fluorocarbonpolymers hydrophilic by means of graft copolymerization of certainmonomers to the hydrophobic fluorocarbon polymer substrate.

DISCLOSURE OF INVENTION

Accordingly the present invention provides an hydrophilicfluoropolymeric diaphragm comprising a fluorine-containing substrate towhich has been radiation co-grafted a mixture of monomers comprising atleast one functional monomer selected from compounds of formula I

    CF.sub.2 ═CF(CF.sub.2).sub.n A

and formula II

    CF.sub.2 ═CF--O--(CFX--CFX).sub.m A

wherein A is carboxyl, alkoxycarbonyl, hydroxyalkoxycarbonyl, cyano,hydroxysulfonyl, fluorosulfonyl, or the group --CO--NR¹ R² wherein R¹and R² are independently selected from hydrogen and C₁ to C₆ alkyl, oneX is fluorine and the other X is selected from chlorine, fluorine and atrifluoromethyl group, n is an integer from 1 to 12, m is an integerfrom 1 to 3; and unsaturated dicarboxylic acids or derivatives thereofcontaining the group of formula III ##STR2## wherein R³ and R⁴ may bethe same or different and represent hydrogen, fluorine, chlorine, andrepresent hydrogen, fluorine, chlorine, and C₁ to C₆ alkyl orhalogenated C₁ to C₆ alkyl group or together form a double bond; and atleast one non-functional monomer selected from the group consisting ofaliphatic vinyl monomers of formula IV

    CY.sub.2 ═CYZ

and aromatic vinyl monomers of formula V ##STR3## wherein Y is hydrogenor fluorine, Z is hydrogen, fluorine or chlorine, and W is hydrogen, C₁to C₆ alkyl, C₂ to C₆ alkenyl, halogenated C₁ to C₆ alkyl or halogenatedC₂ -C₆ alkenyl.

By the term "functional monomers" reference is made to the presence ofthe cation exchange groups in the monomer, such as carboxylic acid andsulphonic acid, or to groups which can be readily converted to cationexchange groups, such as for example carboxylic esters and amides, andntriles. By the term "non-functional monomers" reference is made to theabsence of any ion-exchange group in those monomers.

The molar ratio of co-grafted functional monomer to non-functionalmonomer is in the range of 2:1 to 1:20. Preferably the molar ratio is inthe range from 2:1 to 1:3.

The preferred fluorine-containing substrate is a homopolymer orcopolymer of fluorinated ethylene, especially a homopolymer or acopolymer of vinyliden fluoride, tetrafluoroethylene,chlorotrifluoroethylene or hexafluoropropylene. Typical preferredsubstrates are polytetrafluoroethylene (PTFE);polychlorotrifluoroethylene (PCTFE) and FEP which is the common name forthe copolymer of tetrafluoroethylene and hexafluoropropylene wherein thehexafluoropropylene incorporated in the said copolymer is in the rangeof 3.5-12.5% w/w.

The preferred functional monomers of formula I for use in our processinclude pentafluorobutenoic acid and C₁ to C₆ alkylpentafluorobutenoates such as methyl pentafluorobutenoate and ethylpentafluorobutenoate. A preferred functional monomer of formula II istrifluorovinylsulfonyl fluoride. Preferred functional monomers offormula III are maleic acid, 1,2-difluoromaleic acid,acetylenedicarboxylic acid, and amides, anhydrides, and C₁ to C₆ alkylesters derived from these.

Preferred non-functional monomers of formula IV are tetrafluoroethyleneand chlorotrifluoroethylene. Preferred non-functional monomers offormula V are styrene and its halogenated derivatives, such asα,β,β-trifluorostyrene and divinylbenzene and its halogenatedderivatives, such as α,β,β,α',β',β'-hexafluorodivinylbenzene.

By co-grafted functional and non-functional monomers on a polymericsubstrate we mean that the functional and non-functional monomers formco-polymer side chains on the polymeric substrate such that thefunctional monomer is attached to the substrate by means of anon-functional monomer.

The molecular structures of one type of the side chains of the resins ofthe diaphragm of the present invention may be representeddiagrammatically as follows ##STR4## wherein F and N are the mer unitsderived from the functional and non-functional monomers respectively; Tis a polymer chain-terminating group or polymer substrate; a, b, and dare one or more and c is zero or one or more.

It will be appreciated that this representation does not cover all thepossible configurations of the side chains of the resins of the presentinvention, for example it is also intended that the scope of theinvention shall include side chains having branched configurations,and/or having ordered or random distribution of the functional andnon-functional mer units and/or having more than one type of functionalunit and/or having more than one type of non-functional unit.

In a further embodiment of our invention we provide a process ofpreparing a hydrophilic fluoropolymeric diaphragm, as hereinbeforedefined, which process comprises radiation co-grafting at least onefunctional monomer as hereinbefore defined, and at least onenon-functional monomer as hereinbefore defined, onto a fluoropolymericdiaphragm comprising a polymer or copolymer of a fluorinated ethylene ashereinbefore defined.

It is a characteristic of this process that all of the side chainsformed by the co-grafting process are linked to the substrate polymericdiaphragm by at least one of the non-functional mer unts derived fromthe non-functional monomer. Thus the non-functional monomer provides alinking group whereby the non-functional monomers may be grafted ontothe polymeric substrate.

In this process the functional monomeric material and the non-functionalmonomeric material are mixed in proportions such that the molar ratio ofthe monomers present is in the range of 1:20 to 9:1 respectively.Preferably the molar ratio is in the range of 1:4 to 4:1 and morepreferably, in order to obtain the preferred resins of this invention,the monomeric materials are mixed in nearer to equimolar proportions,i.e. in the range 2:1 to 1:2.

The mixture of monomeric materials has to be in a liquid form and, ifnecessary, a common solvent is used to prepare a solution of them.Commonly one of the monomeric materials itself will provide the liquidphase dissolving the other monomeric material.

Alternatively, with advantage, the solvent used is one which willpenetrate the substrate material and cause it to swell, thereby allowingthe solution of monomers to be absorbed right through the substratematerial. Suitable solvents are, for example, toluene and xylene, andchlorinated hydrocarbons such as trichlorotrifluoroethane and oligomersof tetrafluoroethylene, for example, the tetramer and pentamer oftetrofluoroethylene.

It is also within the scope of this invention for the substrate materialto be preswelled with such solvents prior to the addition of themonomers, the advantage of this procedure being that minimum quantitiesof solvent are used.

Any of the known methods of radiation grafting may be employed. Forexample, the substrate and monomeric materials may be subjected togetherto continuous or intermittent radiation, or the substrate may bepre-irradiated prior to bringing it into contact with the monomericmaterials. Preferably the substrate and monomeric materials areirradiated together; the substrate is immersed in the liquid phasecontaining the mixed monomeric materials and the whole subjected toirradiation by γ-rays, X-rays or electron beam, but preferably byγ-rays.

It is essential for the process of the invention that both thefunctional monomeric material and the non-functional monomeric materialare present together during the grafting process so that the freeradicals generated by the radiation may initiate both the grafting ofnon-functional groups to the substrate and, concurrently, thecopolymerization of the functional and non-functional monomericmaterials to form the chains which characterize the resins of thepresent invention. Preferably the grafting process is carried out in theabsence of oxygen.

In those cases where a derivative of the functional monomer is employedin the grafting process, for example, maleic anhydride, subsequentchemical treatment such as hydrolysis is required to render thedicarboxylate derivative into the active acid form.

It also lies within the scope of our invention to introduce furthercation exchange active groups to the resins, as hereinbefore definedcomprising a substrate, functional groups and non-functional groups. Theadditional active groups are introduced by chemical modification of thegroups already present. Thus, for example, the non-functional groups inthe side chains may be sulphonated and/or carboxylated to give activeresins having enhanced ion exchange capacity and wettability.

In a further embodiment of the process of our invention the functionaland non-functional monomers are radiation co-grafted ontofluoropolymeric substrate material, as hereinbefore defined, inparticulate form such as beads or powder. The co-grafted product is thenfabricated into a diaphragm by any of the conventional means such as bya process of stretching of the resin. This is less preferred than directco-grafting of the pre-formed diaphragm since in the latter case it ispossible to achieve the desired hydrophilicity by grafting the monomericmaterial only to the surfaces of the micropores in the diaphragm thusreducing the quantity of monomeric material required.

BEST MODE OF CARRYING OUT THE INVENTION

The radiation co-grafting of the functional and non-functional linkingmonomers onto the polymeric substrate appears to be governed by twocompeting reactions. One of these is the desired co-grafting of thefunctional monomer on to the non-functional linking monomer which is inturn grafted to the polymeric substrate. The second is thecopolymerization of monomers. Since the rate of copolymerization may begreater than the desired co-grafting, in a preferred embodiment of theprocess of radiation co-grafting the functional and non-functionallinking monomers onto fluorine-containing hydrocarbon polymericsubstrate, as hereinbeforedefined, we provide the improvement comprisingthe addition of at least one polymerization inhibitor and at least onechain transfer agent. In this embodiment higher levels of co-graftingcan be achieved, typically the level of grafting is increased by afactor of three or more. The resins from our improved process havehigher ion exchange capacity, and when such resins are incorporated intodiaphragms for use in electrolytic cells, much better performance isachieved in those cells.

The preferred polymerization inhibitors for use in the process of ourinvention include, for example, quinone inhibitors such asp-benzoquinone, naphthaquinone, and hydroquinone in the presence ofoxygen; inorganic inhibitors such as copper acetate; and compounds suchas 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxide,2,2,6,6-tetramethylpiperazine-N-oxide and chloranil.

The concentration of inhibitor used in the process of our invention isin the range of 0.001 to 2% w/w of the total mixture of functional andlinking monomers and charge transfer agents, preferably in the range of0.01 to 0.5% w/w.

Since the radiation co-grafting is preferably carried out in a liquidmedium it is preferable that the chain transfer agents are also solventsfor the monomers. Preferred chain transfer solvents include, forexample, chloroform, carbon tetrachloride, dimethylformamide andmixtures thereof. Suitable mixtures are for example, chloroform, carbontetrachloride, dimethylformamide and mixtures thereof. Suitable mixturesare for example carbon tetrachloride/chloroform (1:1) and carbontetrachloride/dimethylformamide (1:9). The concentration of monomers inthe chain transfer solvents is in the range of 10-60% w/w, preferably inthe range of 30-50% w/w.

Solid chain transfer agents are less preferred since additional solventsmay be necessary to provide a liquid medium for the radiationco-grafting. If solid chain transfer agents are used the w/w ratio ofsuch transfer agents to the monomers should be in the same range as thatreferred to hereinabove for the preferred chain transfer solvents.

INDUSTRAL APPLICABILITY

Hydrophilic diaphragms, according to the present invention, haveenhanced properties particularly as regards wettability by the liquidspresent in electrolytic cells and therefore they find particularapplication in electrolysis cells. They may also be usefully employed inother electrochemical systems, for example, as separators in batteries,fuel cells and electrolysis cells.

The invention is now illustrated by, but not limited to, the followingexamples in which all ion-exchange capacities are those relating tohighly alkaline conditions, ie all carboxylic acid and sulfonic acidgroups are acting as exchange sites. Unless otherwise stated all partsand percentages are on a weight basis.

EXAMPLE 1

100 grams of commercially available "KEL-F" powder (registered trademark for the homopolymer of chlorotrifluoroethylene), free of additivesand having a particle size about 150 mesh were suspended inmonochlorobenzene (300 ml), containing also 10.0 g (0.096 moles) ofstyrene and 9.4 g (0.096 moles) of maleic anhydride, in a reactionvessel fitted with stirring means, heating means, gas inlet and outletports and condensing means. The suspension was subjected to gammaradiation.

Before and during the gamma radiation a stream of nitrogen gas wasbubbled through the contents of the vessel. The contents of the vesselwere heated to 52.5° C. under agitation and subjected to gamma radiationfor a total of 4.5 hours at a dose rate of 250 krad/hr. The radiatedmixture received the total dose of 1125 krad, after which the radiation,heating and stirring ceased. The grafted resin powder was quantitativelytransferred to a washing column and washed free from unreacted monomers,solvent and unwanted byproducts. Finally, the resin was converted intothe acid form and dried in vacuum oven at 60° C.

The percentage graft, which is calculated by expressing the weightincrease of the resin as a percentage of the weight of grafted resinproduced, was 2.25%. The ion exchange capacity was determined bytitration to be 0.18 meq/g.

Since the ion exchange capacity is related to the amount of thefunctional monomer (maleic anhydride) incorporated by grafting, itprovides a measure of the degree of hydrophilicity obtained by theradiation co-grafting process of the invention. Assuming equimolarproportions of the groups derived from the styrene and from the maleicanhydride monomers in the polymeric side chains grafted onto the "KEL-F"skeleton, the theoretical ion exchange capacity of a resin with a 2.25%graft would be 0.20 mg/g. Examination of the infrared spectrum of theproduct had shown the presence of dicarboxylic acid and styrene in themolecular structure of the resin.

When placed in an aqueous medium containing 25% w/w sodium chloride thehydrolyzed resin was wetted by the solution and sank to the bottom ofthe solution. The fluoropolymer powder before being grafted andhydrolyzed could not be wetted by the solution and formed a whitesurface film on the liquid.

This resin is pressed to form a film and then a microporous diaphragm isprepared by a technique known in the art as calendering wherein the filmis X-Y dimensionally stretched until the desired pore size is reached.The diaphragm is built up to the desired size by successive layers ofstretched film. The hydrophilicity of the resin is unchanged duringthese processes of fabrication so that the hydrophilic properties areconferred on the final diaphragm.

EXAMPLES 2 to 5

Graft copolymers of styrene-maleic anhydride to "KEL-F" powder,according to the present invention, were made by the method described inExample 1, except that different total monomer concentrations (keepingthe molar ratios of monomers constant) were used to produce variouslevels of grafts, resulting in various exchange capacities of thegrafted product resin as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Example          2      3       4     5                                       ______________________________________                                        Concentration of monomers                                                     in monochlorobenzene                                                          Styrene g/l      21.0   53.3    126.6 286.6                                   Maleic anhydride g/l                                                                           19.7   50.7    120.0 266.6                                   Percentage graft %                                                                             1.25   3.12    5.06  8.01                                    Theoretical ion exchange                                                                       0.11   0.28    0.44  0.67                                    capacity meq/g                                                                Measured ion exchange                                                                          0.09   n.d.    0.40  n.d.                                    capacity meq/g                                                                ______________________________________                                    

Infra-red analysis confirmed the presence of dicarboxylic acid andstyrene in the molecular structure of the grafted resin products. All ofthe hydrolyzed resins were wettable by 25% sodium chloride solution.

These resins are formed into microporous diaphragms by the methoddescribed in Example 1.

EXAMPLES 6 to 8

These examples illustrate the products of the present invention having adifferent fluorine containing hydrocarbon polymer substrate to those ofexample 1 to 5.

In these examples styrene and maleic anhydride were graft copolymerizedby the process according to the invention, using the conditionsdescribed in Example 1, to "FLUON" powder, which is a homopolymer oftetrafluoroethylene ("FLUON" is a registered trade mark of ImperialChemical Industries Ltd). The percentage grafts and ion exchangecapacities of the product resins in their acid form obtained usingvarious monomer concentrations are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        Example           6        7      8                                           ______________________________________                                        Concentration of monomers                                                     in monochlorobenzene                                                          styrene g/l       21.0     33.7   143.3                                       maleic anhydride g/l                                                                            19.7     31.7   135.0                                       Percentage graft %                                                                              0.75     1.22   3.12                                        Theoretical ion exchange                                                                        0.06     0.11   0.28                                        capacity                                                                      Measured ion exchange                                                                           n.d.     0.08   n.d.                                        capacity meq/g                                                                ______________________________________                                          dicarboxylic acid and styrene in the molecular structure of the grafted     resin products.

These resins are formed into microporous diaphragms by the methoddescribed in Example 1.

EXAMPLES 9 TO 12

Treatment of samples of products from some of the previous examples by aknown process for substituting sulphonate groups into the styrene groupsproduced sulphonated resins having, the ion-exchange capacities given inTable 3.

                  TABLE 3                                                         ______________________________________                                        Example Sulphonated  Ion-exchange capacity meq/g                              No      product from Theoretical                                                                              Measured                                      ______________________________________                                         9      Example 1    0.31       0.26                                          10      Example 2    0.17       0.13                                          11      Example 4    0.64       0.52                                          12      Example 7    0.17       0.13                                          ______________________________________                                    

The ion exchange capacities of these sulphonated resins were all greaterthan their non-sulphonated analogues.

EXAMPLE 13

100 grams of "KEL-F" powder similar to that used in Example 1 weresuspended in 300 ml of a solution of maleic anhydride andtetrafluoroethylene in toluene. The solution contained 0.7 g/kg ofmaleic anhydride and 0.7 g/kg of tetrafluoroethylene.

The suspension was frozen by immersing its container in liquid nitrogen.It was degassed and allowed to regain room temperature. The degassingprocedure was repeated three times and the container sealed.

The solution in the sealed container was heated to 70° C. and held atthat temperature for 24 hours. The container and its contents weresubjected to γ-radiation for a total of 50 hours at a dose rate of 100krad/hr.

After irradiation the container was again immersed in liquid nitrogen, anecessary precaution with tetrafluoroethylene to freeze the suspensionbefore the container was opened. The powder was washed free of unreactedand ungrafted homopolymer monomers. It was found that 20% graft hadtaken place. The powder was pressed to form a film which was thenhydrolyzed. The ion exchange capacity of the hydrolyzed film wasdetermined to be 0.64 meq/g. On the basis of the percentage graft andthe ion exchange capacity it was calculated that the molar ratio offunctional to non-functional groups in the side chains grafted on the"KEL-F" substrate was approximately 1:3.

EXAMPLE 14

In this example, the advantage of using a swelling solvent isdemonstrated.

4 grams of "KEL-F" powder similar to that used in Example 1 wereimmersed in hot xylene. The powder swelled and absorbed an amount ofxylene equal to approximately 7% of its own weight. Excess xylene wasremoved. An equimolar mixture of styrene and maleic anhydride was addedto the swollen powder. After 12 hours the excess liquid phase wasdecanted off and the swollen powder with absorbed monomers wasirradiated under nitrogen with radiation at a level of 80 krad/hour for24 hours.

After removal of any homopolymer formed and of any unreacted monomers,the powder was hydrolyzed.

By the weight increase it was calculated there had been a 10.0% graft.

The ion exchange capacity of the resin was determined to be 1.1 meq/gwhich, assuming the styrene and maleic anhydride were grafted on inequimolar proportions, would indicate a 11.0% graft.

This resin is formed into a microporous diaphragm by the methoddescribed in Example 1.

EXAMPLES 15-26

These examples demonstrate the use of γ-radiation of perfluorinatedfilms or powders in the presence of unsaturated perfluorinated monomerscontaining functional or functionable ion-exchange groups.

A sample of each monomer (5.0 gm) was placed in a glass reaction vessel,and (5.0 gm) of unsaturated perfluorinated monomer or monomer mixtureadded. The contents of the reaction vessel were frozen in liquidnitrogen and placed under vacuum to remove the air present in thesystem.

After thorough evacuation, the vacuum pump was disconnected and thecontents allowed to thaw and reach room temperature. This process,hereinafter referred to as degassing, was repeated three times beforesealing the reaction vessel.

Using this technique the contents of the reaction vessel were in avirtually oxygen-free atmosphere. Furthermore, the samples prepared,using this method, were then allowed to equilibrate at selectedtemperatures for a period of twenty-four hours.

After this time, the reaction vessel was transferred to an irradiationcell room and exposed for 120 hours to γ-rays emanating from a Cobalt-60source of an intensity equivalent to 10 krad/hr.

In some experiments a solvent, trifluorotrichloroethylene, was added inthe concentration shown in Table 4. The simultaneously irradiatedcontents of the reaction vessel received a total absorbed dose of 1.2Mrad after termination of the irradiation, the contents of the glassreaction vessel were frozen in liquid nitrogen prior to opening thereaction vessel. The grafted substrate (film or powder) was washed freeof unreacted monomer and homopolymer with a suitable solvent and driedin a vacuum oven at 60° C. to constant weight.

The percentage of graft (expressed as the weight increase of the film asa percentage of the weight of the grafted film), the infrared spectra ofthe grafted film (carbonyl absorption at frequency of 1795 cm⁻¹) and theion exchange capacity (expressed as meq/g) are used to characterize themodified perfluorinated substrate produced by γ-radiation.

The results presented in Table 4 indicate that the perfluorinatedhydrophobic substrate was grafted with unsaturated perfluorinatedmonomer having functional or functionable ion exchange groups.

In the Tables and description, the following abbreviations are used:

TFE=tetrafluoroethylene

PTFE=polytetrafluoroethylene

PEP=copolymer of tetrafluoroethylene and hexafluoropropylene

PFBA=perfluorobutenoic acid

MPFB=methyl perfluorobutenoate

MAA=methacrylic acid

IEC=ion exchange capacity

The sizes given are in microns and refer to thickness, in the case offilms, and particle size in the case of powders.

These films and powders are formed into microporous diaphragms by themethod described in Example 1.

                  TABLE 4                                                         ______________________________________                                                         Monomer                                                                       Con-        Solvent                                          Ex-              centration  concen-      IEC                                 am-  Sub-        % w/w       tration                                                                              %     meq/                                ple  strate   Size   PFBA  MPFB  % w/w  Graft g                               ______________________________________                                        15   PTFE     175    100   --    --     2.5   0.13                                 film                                                                     16   PTFE     2-5    100   --    --     3.6   0.21                                 powder                                                                   17   PTFE     175    --    100   --     1.4   --                                   film                                                                     18   PTFE     2-5    --    100   --     2.2   0.11                                 powder                                                                   19   FEP      125    100   --    --     1.6   --                                   film                                                                     20   FEP      125    --    100   --     1.2   --                                   film                                                                     21   PTFE     175     30   --    70     3.1   0.15                                 film                                                                     22   PTFE     175    --     30   70     2.4   0.11                                 film                                                                     23   FEP      125     30   --    70     2.5   0.13                                 film                                                                     24   FEP      125    --     30   70     1.8   --                                   film                                                                     25   PTFE     2-5     30   --    70     3.9   0.20                                 powder                                                                   26   PTFE     2-5    --     30   70     2.3   0.11                                 powder                                                                   ______________________________________                                    

EXAMPLES 27-38

In these examples the conditions used in the preceding examples wererepeated except that the radiation grafting was carried out at thetemperatures shown in Table 5 along with the results obtained.

                                      TABLE 5                                     __________________________________________________________________________                Monomer                                                                       Con-     Solvent                                                              centration                                                                             concen-                                                  Ex- Sub-    % w/w    tration                                                                            %   Temp                                            ample                                                                             strate                                                                            Size                                                                              PFBA                                                                              MPFB % w/w                                                                              Graft                                                                             °C.                                                                         meq/g                                      __________________________________________________________________________    27  PTFE                                                                              175 100 --   --   3.1 35   --                                             film                                                                      28  PTFE                                                                              175 100 --   --   3.9 80   0.21                                           film                                                                      29  PTFE                                                                              175 --  100  --   2.1 35                                                                            --                                                  film                                                                      30  PTFE                                                                              175 --  100  --   3.0 80   0.13                                           film                                                                      31  FEP 125 100 --   --   2.2 35   --                                             film                                                                      32  FEP 125 100 --   --   3.1 80   0.15                                           film                                                                      33  FEP 125 --  100  --   1.8 35   --                                             film                                                                      34  FEP 125 --  100  --   2.6 80   0.11                                           film                                                                      35  PTFE                                                                              175  30 --   70   3.5 50   0.17                                           film                                                                      36  PTFE                                                                              175 --   30  70   3.0 50   --                                             film                                                                      37  FEP 125  30 --   70   3.0 50   0.15                                           film                                                                      38  FEP 125 --   30  70   2.5 50   --                                             film                                                                      __________________________________________________________________________

EXAMPLES 39-43

In these examples the general conditions of Examples 15-26 were usedexcept that the temperature of the mixture during radiation was 50° C.and the solvent was varied as shown in Table 6 along with the resultsobtained.

In each case the substrate was PTFE film of 175 micron thickness.

                  TABLE 6                                                         ______________________________________                                             Monomer                Solvent                                           Ex-  Concentration          concen-                                           am-  % w/w                  tration                                                                              %     IEC                                  ple  PFBA    MPFB    Solvent  % w/w  Graft meq/g                              ______________________________________                                        39   30      --      CF.sub.2 ClCFCl.sub.2                                                                  70     3.4   0.17                               40   30      --      water    70     3.8   0.20                               41   30      --      methanol 70     1.2   --                                 42   30      --      carbon   70     1.8   --                                                      tetra-                                                                        chloride                                                 43   --      30      carbon   70     2.1   0.08                                                    tetra-                                                                        chloride                                                 ______________________________________                                    

The grafted films are formed into microporous diaphgragms by the methoddescribed in Example 1.

EXAMPLES 44-49

These examples used the conditions of Examples 15-26 and illustrate theuse of a mixture of two functional monomers.

                                      TABLE 7                                     __________________________________________________________________________                 Monomer                                                          Ex-          Concentration                                                                             Solvent                                              am-          % w/w       Concen-                                                                            %    IEC                                        ple                                                                              Substrate                                                                           Size                                                                              PFBA                                                                              MPFB                                                                              MAA tration                                                                            Graft                                                                              meq/g                                      __________________________________________________________________________    44 PTFE film                                                                           175 25  --  25  50   17.6 0.78                                       45 PTFE film                                                                           175 --  25  25  50   20.0 0.44                                       46 PTFE  2-5 25  --  25  50   28.0 0.95                                          powder                                                                     47 PTFE  2-5 --  25  25  50   31.2 1.12                                          powder                                                                     48 FEP film                                                                            125 25  --  25  50    9.5 0.41                                       49 FEP film                                                                            125 --  25  25  50   12.1 0.35                                       __________________________________________________________________________

EXAMPLE 50

This example illustrates the use of a grafting inhibitor α-pinene.

FEP film (2.0 g) was placed in a glass reaction vessel and PFBA (9.0 g),α-pinene (0.12 g, 0.5% total concentration), "Arklone" P (9.0 g), water(3.0 g) and ammonium perfluorooctanoate (0.025 g, 0.17% totalconcentration) were added to the vessel. ("Arklone" is a registeredtrade mark for 1,1,2-trichloro-1,2,2-trifluoroethylene).

The mixture was frozen in liquid nitrogen, the air was evacuated andcontents allowed to come to room temperature. This process of degassingwas repeated three times; then TFE (3.0 g, free from any inhibitor) wascharged into the reaction vessel at liquid nitrogen temperature. Theglass vessel was sealed and kept at room temperature overnight. It wasthen irradiated at 10 krad/hr for 120 hours at ambient temperature. Themixture received a total dose of 1.2 Mrad and it was frozen with liquidnitrogen again, prior to opening the glass vessel. The grafted film wascollected, washed free of copolymers and unreacted monomers and dried invacuum oven at 60° C.

The percentage graft was 47%. The infra-red spectrum of the grafted filmshowed the carbonyl absorption frequency to by 1795 Cm⁻¹ and its ionexchange capacity determined by titration was 0.2 meq/g.

This film is formed into a microporous diaphragm by the method describedin Example 1.

EXAMPLES 51-55

The procedure of Example 50 was repeated except that the concentrationof the inhibitor α-pinene was varied as shown in Table 8.

                  TABLE 8                                                         ______________________________________                                        Ex-     Concentration of %       IEC                                          ample   α-pinene % w/w                                                                           Graft   meq/g                                        ______________________________________                                        51      0.00             61      0.11                                         52      0.05             58      --                                           53      0.1              59      0.10                                         54      1.0              35      0.12                                         55      2.5              10      0.04                                         ______________________________________                                    

EXAMPLES 56-58

The procedure of Example 36 was repeated except that the "Arklone P"solvent was replaced by an equal weight of solvent or solvent mixture asshown in Table 9.

                  TABLE 9                                                         ______________________________________                                        Ex-                       %        IEC                                        ample   Solvent           Graft    meq/g                                      ______________________________________                                        56      "Arklone" P/chloroform                                                                          60.0     0.02                                               (1:1)                                                                 57      Carbon tetrachloride/                                                                           60.9     --                                                 Chloroform (1:1)                                                      58      Chloroform        60.0     --                                         ______________________________________                                    

EXAMPLES 59-66

These examples demonstrate the effect of γ radiation on a perfluorinatedmicroporous diaphragm in the presence of unsaturated perfluorinatedmonomers containing functional or functionable ion exchange groups.

The selected microporous diphragm "Goretex" (registered trade mark ofGore Associates) has a pore size around ˜100μ and thickness of ˜1500μ.

The "Goretex" diaphragm has pronounced hydrophobic properties but by theprocess of the invention a modified hydrophilic diaphragm was obtained.The method of Examples 15-26 was used and the results are given in Table10. In each case a dose rate of 15 krad/hr was used to give a total doseof 1.25 Mrad. The solvent used was "Arklone" P.

                                      TABLE 10                                    __________________________________________________________________________            Monomer      Solvent                                                  Ex-                                                                              Substrate                                                                          Concentration                                                                              Concen-                                                  am-                                                                              and  % w/w        tration                                                                            Temp                                                                              %    IEC                                        ple                                                                              size PFBA                                                                              MPFB MAA % w/w                                                                              °C.                                                                        Graft                                                                              meq/g                                      __________________________________________________________________________    59 "Goretex"                                                                          100 --   --  --   35  4.8  0.28                                          1500                                                                       60 "Goretex"                                                                          --  100  --  --   35  5.1  0.25                                          1500                                                                       61 "Goretex"                                                                          100 --   --  --   80  5.6  0.31                                          1500                                                                       62 "Goretex"                                                                          --  100  --  --   80  5.9  0.29                                          1500                                                                       63 "Goretex"                                                                           30 --   --  70   35  3.8  0.20                                          1500                                                                       64 "Goretex"                                                                          --   30  --  70   35  4.1  0.18                                          1500                                                                       65 "Goretex"                                                                           25 --   25  50   35  26.3 0.91                                          1500                                                                       66 "Goretex"                                                                          --   25  25  50   35  30.5 1.06                                          1500                                                                       __________________________________________________________________________

We claim:
 1. An hydrophilic fluoropolymeric microporous diaphragmcomprising a fluorine-containing polymeric substrate to which has beenradiation co-grafted a mixture of monomers comprising at least onefunctional monomer selected from the group consisting of compounds offormula I

    CF.sub.2 ═CF(CF.sub.2).sub.n A,

and formula II

    CF.sub.2 ═CF--O--(CFX--CFX).sub.m A

wherein A is carboxyl, alkoxycarbonyl, hydroxyalkoxy, carbonyl, cyano,hydroxsulfonyl, fluorosulfonyl, or the group --CO--NR¹ R² wherein R¹ andR² are independently selected from hydrogen and C₁ to C₆ alkyl, one X isfluorine and the other X is selected from chlorine fluorine and atrifluoromethyl group, n is an integer from 1 to 12, m is an integerfrom 1 to 3, and unsaturated dicarboxylic acids or derivatives thereofcontaining the group of formula III ##STR5## wherein R³ and R⁴ areindependently selected from hydrogen, fluorine, chlorine, and C₁ to C₆alkyl or halogenated C₁ to C₆ alkyl or together form a double bond; andat least one non-functional monomer selected from the group consistingof aliphatic vinyl monomers of formula IV and aromatic vinyl monomers offormula V ##STR6## wherein Y is hydrogen or fluorine, Z is hydrogen,fluorine or chlorine, and W is hydrogen, C₁ to C₆ alkyl, C₂ to C₆alkenyl, halogenated C₁ to C₆ alkyl or halogenated C₂ to C₆ alkenyl; andwherein the molar ratio of co-grafted functional monomer to co-graftednon-functional monomer is in the range of 2:1 to 1:20.
 2. A microporousdiaphragm according to claim 1 wherein the molar ratio of saidfunctional monomer to said non-functional monomer is in the range of 2:1to 1:3.
 3. A microporous diaphragm according to claim 1 or claim 2wherein the fluorine-containing polymeric substrate is a homopolymer orcopolymer of a fluorinated ethylene.
 4. A microporous diaphragmaccording to claim 3 wherein the fluorinated ethylene is vinylidenefluoride.
 5. A microporous diaphragm according to claim 3 wherein thefluorinated ethylene is tetrafluoroethylene.
 6. A microporous diaphragmaccording to claim 3 wherein the fluorinated ethylene ischlorotrifluoroethylene.
 7. A microporous diaphragm according to claim 3wherein the copolymer comprises hexafluoropropylene units.
 8. Amicroporous diaphragm according to claim 1 wherein thefluorine-containing polymeric substrate is polytetrafluoroethylene.
 9. Amicroporous diaphragm according to claim 1 wherein thefluorine-containing polymeric substrate is polychlorotrifluoroethylene.10. A microporous diaphragm according to claim 1 wherein thefluorine-containing polymeric substrate is a copolymer oftetrafluorethylene and hexafluoropropylene wherein thehexafluoropropylene incorporated in the said copolymer is in theconcentration range of 3.5-12.5% w/w.
 11. A microporous diaphragmaccording to claim 1 wherein the compounds of formula I and II arepentafluorobutenoic acid, C₁ to C₆ alkyl pentafluorobutenoates andtrifluorovinylsulfonyl fluoride.
 12. A microporous diaphragm accordingto claim 11 wherein the said C₁ to C₆ alkyl pentafluorobutenoates aremethyl pentafluorobutenoate and ethyl pentafluorobutenoate.
 13. Amicroporous diaphragm according to claim 1 wherein the compounds offormula III are maleic acid, 1,2-difluoromaleic acid, acetylenedicarboxylic acid and anhydrides, amides and C₁ to C₆ alkyl estersthereof.
 14. A microporous diaphragm according to claim 1 wherein thenon-functional monomers of formula IV are tetrafluoroethylene andchlorotrifluoroethylene.
 15. A microporous diaphragm according to claim1 wherein the non-functional monomers of formula V are styrene, α, β,β-triflurostyrene, divinylbenzene and α, β, β, α', β',β'-hexafluorodivinylbenzene.
 16. A process of preparing an hydrophilicfluoropolymeric microporous diaphragm which process comprises radiationco-grafting onto a fluorine-containing polymeric diaphragm a mixture ofmonomers comprising at least one functional monomer selected from thegroup consisting of compounds of formula I

    CF.sub.2 ═CF(CF.sub.2).sub.n A,

and formula II

    CF.sub.2 ═CF--O--(CFX--CFX).sub.m A

wherein A is carboxyl, alkoxycarbonyl, hydroxyalkoxy, carbonyl, cyano,hydroxysulfonyl, fluorosulfonyl, or the group --CO--NR¹ R² wherein R¹and R² are independently selected from hydrogen and C₁ to C₆ alkyl, oneX is fluorine and the other X is selected from chlorine fluorine and atrifluoromethyl group, n is an integer from 1 to 12, m is an integerfrom 1 to 3, and unsaturated dicarboxylic acids or derivatives thereofcontaining the group of formula III ##STR7## wherein R³ and R⁴ areindependently selected from hydrogen, fluorine, chlorine, and C₁ to C₆alkyl or halogenated C₁ to C₆ alkyl or together form a double bond; andat least one non-functional monomer selected from the group consistingof aliphatic vinyl monomers of formula IV

    CY.sub.2 ═CYZ

and aromatic vinyl monomers of formula V ##STR8## wherein Y is hydrogenor fluorine, Z is hydrogen, fluorine or chlorine, and W is hydrogen, C₁to C₆ alkyl, C₂ to C₆ alkenyl, halogenated C₁ to C₆ alkyl or halogenatedC₂ to C₆ alkenyl; and wherein the molar ratio of functional monomer tonon-functional monomer is in the range of 1:20 to 9:1.
 17. A processaccording to claim 16 wherein the molar ratio is in the range of 1:4 to4:1.
 18. A process according to claim 16 wherein the molar ratio is inthe range of 1:2 to 2:1.
 19. A process according to claim 16 wherein thematerial comprising the diaphragm and the mixture of monomers aresubjected together to irradiation by any one form of radiation selectedfrom the group consisting of γ-rays, X-rays and electron beams.
 20. Aprocess according to claim 16 wherein the mixture of monomers isdissolved in a solvent capable of swelling the diaphragm.
 21. A processaccording to claim 16 wherein before addition of the monomers thediaphragm is treated with a solvent capable of swelling the diaphragm.22. A process according to claim 20 wherein the solvent is selected fromthe group consisting of toluene, xylene, trichlorotrifluoroethane andoligomers of tetrafluoroethylene.
 23. A process according to claim 16wherein the mixture of monomers additionally comprises at least onepolymerization inhibitor and at least one chain transfer agent.
 24. Aprocess according to claim 23 wherein the polymerization inhibitor isselected from the group consisting of p-benzoquinone, naphthaquinone,hydroquinone in the presence of oxygen, copper acetate,2,2,6,6-tetramethyl-4-oxopiperidine-1-oxide,2,2,6,6-tetramethylpiperazine-N-oxide, and chloranil.
 25. A processaccording to claim 23 wherein the polymerization inhibitor is in theconcentration range of 0.001 to 2% w/w of the total mixture of monomersand charger transfer agent.
 26. A process according to claim 25 whereinthe concentation range is 0.01 to 0.5% w/w.
 27. A process according toclaim 24 wherein the chain transfer agent is a solvent selected from thegroup consisting of chloroform, carbon tetrachloride, dimethylformamideand mixtures thereof.
 28. A process according to claim 24 wherein theconcentration of the monomers in the chain transfer solvent is in therange of 10 to 60% w/w.
 29. A process according to claim 28 wherein thesaid range is 30-50% w/w.